The present invention relates to logging tools and particularly to a borehole televiewer tool (BHTV) such as described in U.S. Pat. No. 3,369,626. This patent describes a logging tool in which a rotating acoustical transducer is pulsed to produce acoustic waves which travel through the borehold fluid and are reflected from the wall of the borehole back to the transducer. Various characteristics of the reflected signal are recorded and displayed to provide an indication of the surface of the borehole. For example, the amplitude of the reflected signal can be displayed on a cathode ray tube which is periodically photographed to provide a picture of the wall of the borehole unfolded to a flat surface. The tool has proven useful in locating fractures in formations penetrated by the borehole, as well as locating corrosion in casing which has been installed in the borehole. The tool has also been adapted to provide a caliper type measurement of the borehole so that enlarged sections of the borehole can be located. A display system that incorporates the above features is described in U.S. Pat. No. 4,463,378.
While the tool has proven useful it does have limitations when run in boreholes containing heavy weight drilling muds. Heavy weight muds are used in the many areas, particularly in the Gulf Coast area of the United States, where the drilling of geopressured formations is common. Heavy weight muds tend to absorb and scatter the acoustic signal and limit the ability of the tool to investigate the borehole wall.
A possible solution to the problem is to use lower frequencies to provide acoustic waves that can better penetrate the drilling mud. The use of lower frequencies requires larger transducers to focus the acoustic energy. Since the transducer is mounted in the tool and rotated to scan the complete borehole wall, there is a physical limitation on the size of the transducer that can actually be mounted in the tool. If lower frequencies are used without enlarging the transducers, the beam width defined as the full width at half height of the primary lobe becomes very large, increasing the spot size and reducing the resolution.